The present description relates to control for an electrically driven supercharger of an internal combustion engine.
Conventionally, a turbocharger or a mechanical supercharger is used to increase or “boost” engine torque. A mechanical supercharger may be powered by the engine through a mechanical connection such as a belt drive mechanism coupling the engine crankshaft to the supercharger's shaft. A turbocharger is powered by engine exhaust gas energy that flows through a turbine that is arranged in the engine exhaust passage. The turbine is coupled to a compressor that pressurizes air entering the engine. Consequently, the speed of the mechanical supercharger is usually in proportion with the speed of the engine. Likewise, engine speed affects the flow through an engine and therefore influences the speed of the turbine. Supercharging efficiency of the mechanical supercharger or of the turbocharger may be affected by the engine speed. In particular, it may be deteriorated at a lower engine speed.
To address this issue, there is known and presented an electrically driven supercharger or an electric supercharger, for example, in U.S. Pat. No. 6,684,863. Since an electric motor drives the supercharger with electricity supplied from a battery, the engine speed may not affect the operation of the supercharger. When the “boost” is required, the electric supercharger is controlled to rotate at a rated operating speed, such as 60,000 rpm. When the boost is not desired, such as during a lower load condition, the intake air bypasses the supercharger through a bypass passage, thereby making it unnecessary to drive the supercharger. When boost is desired, the bypass passage is closed and the supercharger compresses the incoming air. However, at lower load conditions, the electric supercharger is not controlled to stop because stopping the supercharger would increase the time necessary for the supercharger to reach a desired speed when the engine torque demand is transitioned to a higher load condition where the boost is desired. Rather, the supercharger is controlled to rotate at an idle speed that may be up to 20,000 rpm. The supercharger idle speed is set according to various conditions that include the probability of transitioning to a higher load and the state of charge of the battery.
During a transition from a non-boost condition to a boost condition, the speed of the supercharger is increased from the idle speed to the target speed using feedback control, thereby supplying maximum electricity or inrush electric current to the electric motor from the idle speed to the target speed. At that time, the inrush of electric current may exceed electric generation capacity of electric generator or alternator, thereby consuming electric charge of the battery. In the '863 patent, the supercharger idle speed is set lower as the battery charge is lower. When the battery is degraded, for example, due to a long time use, the idle speed is set lower and the feedback control of the supercharger speed increases the inrush current. Such an increase of inrush current may cause a further degradation of the battery. This in turn, may make it more difficult to start the engine when relatively large amounts of electricity are needed, for example.
The inventors herein have recognized the problem of the above prior art and have developed a method to improve the control on the electric supercharger.